Yes, It is possible to edit and cancel an appointment that has already started. To cancel the appointment, you must first close the tab with the call from your browser. Then go to the appointment booking from the calendar, click on the three dots in the upper right corner of the appointment and select Cancel. Please note that the Cancel option disappears if you exit the appointment through the door icon in the call; This is only possible if you leave the appointment by closing the tab in the browser.
To edit the appointment during an ongoing or finished videocall go to the calendar and click on the "pen" icon. This will give you the possibility to edit participants and date/time of the appointment when needed.